Japanese Patent Application Laid-open No. 2004-3584 discloses a rotary damper in which a valve body comprising a leaf spring is disposed on a vane or a division wall (hereinafter called “vane or the like”), and a flow rate of viscous fluid passing through a fluid passage can be adjusted in accordance with variation in load by the valve body (see paragraphs 0042 to 0053 and FIGS. 7 to 11 of this publication).
One surface of the valve body of the rotary damper is deflected such as to project, the one surface receives fluid pressure and is deformed, and a size of a flow path through which fluid passes can be changed. According to the rotary damper, since the flow rate of fluid passing through the fluid passage can be limited in accordance with a load by the valve body, it is possible to change the braking force in accordance with variation in load.
According to the valve body, however, since a straight distance between an opening of the fluid passage closed with the valve body and the other surface of the valve body opposed to the opening is short, there is a problem that the load range which is to be coped is small. If the load reaches a given value, there is also a problem that the valve body is abruptly deformed and the fluid passage is closed. Further, since the valve body is bent, if the valve bodies are mass-produced, it is not easy to form the valve bodies into uniform shape, and there is a problem that a slight error in shape results in great influence on braking characteristics.
When the valve body is disposed on the vane or the like, it is necessary that the vane or the like has a width (length in the circumferential direction) to a certain extent. When an outer diameter of a casing is large, it is easy to secure the width of the vane or the like on which the valve body can be disposed, but in this case, there is a problem that the installation space of the rotary damper is inevitably increased. On the other hand, to reduce the installation space of the rotary damper, if the outer diameter of the casing is reduced, the width of the vane or the like is reduced to secure the rotation angle of the vane or the like, and there is a problem that it becomes difficult to provide the valve body.    Patent Document 1: Japanese Patent Application Laid-open No. 2004-3584